The invention relates to an actuator of a paper or board machine comprising a motor; a shaft driven by the motor, the position of the actuator in the linear direction being adjusted by rotating the shaft by the motor; and a sensor arranged to measure the rotation of the shaft and to generate a pulse output proportional to the rotation of the shaft to define the position of the actuator in the linear direction.
Actuators are used, for example, to control the flow of different media, such as water and steam, in different processes. The actuators control a valve, nozzle or the like supplying the medium. The actuator can be a motor, cylinder or the like, which can be controlled in many different ways, for example mechanically, electrically, hydraulically or pneumatically.
U.S. Pat. No. 4,662,398 discloses a valve controlling the flow of steam and air to the steam box of an apparatus for drying a fiber web. The valve is controlled by a stepping motor. The control arrangement of the stepping motor is very complicated, and it is uncertain whether the motor will operate reliably in the very difficult process conditions of a paper machine. Further, the positioning data of the valve, defined solely on the basis of the control of the stepping motor, is rather unreliable.
DE Utility Model 29 701 674 discloses an actuator comprising a valve that is controlled by a brushless electrical motor. The position of the rotor of the brushless motor is measured by means of Hall sensors, and so the position of the actuator is also defined on the basis of the pulses generated by the Hall sensor. The publication teaches that the Hall sensor is also particularly used to effect the rotating motion of the motor. The control system of the brushless motor is complicated, and the rotation of the rotor is also difficult and complicated to measure. It is thus out of the question to use the sensor defining the position of the rotor and the brushless motor in the processing conditions of a paper machine.
FI Patent 85 731 discloses actuators whose spindles are adjusted by stepping motors. The position of the spindles is measured by LVDT sensors. The control arrangement of the stepping motor is complicated and cumbersome. The LVDT sensor endures the process conditions of a paper machine rather poorly, and it is susceptible to malfunction. The signal generated by the LVDT sensor is also very weak, and for example due to great temperature dependence, the accuracy of the sensor is rather poor.
GB Patent Application 2 225 415 discloses a fluid flow valve controlled by a stepping motor. The position of the valve can be measured by an optical shaft encoder. The optical encoder requires a complicated and sensitive electronic arrangement, which is difficult to arrange in conjunction with a paper machine: the optical encoders do not endure the process conditions of a paper machine.
The actuator of the invention is characterized in that at least one detection point is arranged in conjunction with the shaft, and that the apparatus comprises at least one Hall sensor, which generates a pulse when the detection point of the shaft revolves past the Hall sensor.
The essential idea of the invention is that a detection point is arranged in conjunction with the shaft of the actuator, and that a Hall sensor is arranged in the apparatus, the Hall sensor outputting a pulse when the shaft rotates and the detection point passes the Hall sensor. When the shaft of the actuator rotates, the position of the actuator can be defined on the basis of the pulses generated by the Hall sensor. In a preferred embodiment, the detection point arranged in conjunction with the shaft is provided by a magnet, the motion of which is detected by the Hall sensor when the shaft rotates. In a second preferred embodiment, the measurement is taken at the shaft of the motor of the actuator. In a third preferred embodiment, the motor of the actuator is a synchronous motor.
The advantage of the invention is that the position of the actuator can be measured in an accurate and simple way using an apparatus that endures even hard process conditions. When magnets are arranged on the shaft, the signal generated by the Hall sensor is very reliable. When the measurement is taken at the shaft of the motor, great measurement accuracy is achieved. The use of a synchronous motor makes the actuator simple and reliable, and easy to control.